1. Technical Field
The present invention relates to photo transistors, and more particularly to a driving method of a photo transistor, and a photo sensor and a flat panel display using the method.
2. Description of Related Art
Photo transistors and photo sensors using such photo transistors have the advantage of high sensitivity and have thus been widely used in various electronic devices, such as flat panel displays.
Referring to FIG. 6, a typical photo sensor 10 includes a photo transistor 11, a resistor 12, a first voltage terminal 13, a second voltage terminal 14, and a signal output terminal 15. The photo transistor 11 is an amorphous silicon (a-Si) thin film transistor (TFT), and includes a gate electrode 111, a source electrode 112, a drain electrode 113, and an a-Si layer (not shown). The gate electrode 111 is connected to the first voltage terminal 13. The source electrode 112 is connected to the second voltage terminal 14. The drain electrode 113 is connected to the signal output terminal 15 and to one end of the resistor 12. The other end of the resistor 12 is grounded.
The first voltage terminal 13 and the second voltage terminal 14 output a first voltage and a second voltage to the gate electrode 111 and the source electrode 112 respectively, to drive the photo transistor 11. The first voltage is five volts (5V) direct current (DC) voltage, and the second voltage is 1V DC voltage.
When the photo transistor 11 is exposed to light, the a-Si layer generates a plurality of photocarriers so as to produce photocurrent between the source electrode 112 and the drain electrode 113. In operation, with variations in the light intensity in external environments, the quantity of photocarriers and photocurrent correspondingly change. Thereby, a resistance between the source electrode 112 and the drain electrode 113 varies, and a signal output by the signal output terminal 15 correspondingly varies. As a result, the variations of light intensity can be measured.
Due to the first voltage used for driving the gate electrode 111 of the photo transistor 11 being a positive DC voltage, a plurality of electrons attracted by the first voltage reside in the a-Si layer of the photo transistor 11. These electrons can restrict the motion of the photocarriers, such that the photocurrent and the signal output by the signal output terminal 15 are weakened. Thus, the reliability of the photo sensor 10 using DC voltage to drive the photo transistor 11 is somewhat low.
What is needed is a driving method for a photo transistor which can overcome the limitations described, and a photo sensor using the driving method and a flat panel display using the photo sensor.